In recent years, the heat generation amount of electronic components has further increased with high functionality of electronic components for use in electronic devices. Then, in order to efficiently dissipate the heat generated in the electronic components, a metal plate is sometimes used for a core material of the wiring substrate.
Japanese Unexamined Patent Application Publication No. 200-353584 describes a wiring substrate having a metal plate serving as a core material and a wiring layer containing resin applied to both front and back surfaces of the metal plate and a conductive layer.
In general, the coefficient of thermal expansion is different between metal and resin. When heat is applied to the wiring substrate, the thermal expansion difference in the planar direction of the metal plate and an insulator becomes large. Therefore, stress is added to the interface of the metal plate and the wiring layer, and there is a possibility that the wiring layer is separated from the metal plate due to the stress.
As a result, disconnection occurs in the conductive layer of the wiring substrate, so that the electrical reliability of the wiring substrate easily decreases. Therefore, it has been required to increase the electrical reliability of the wiring substrate.